ASK THE EXPERTS: Have Dam, Need Hydro System

I’ve been reading and studying many of your articles on microhydro. Most are tapping energy from small streams—I have a dam.

There’s roughly 10 to 15 feet of head from the intended intake to just below the dam’s discharge culvert to the turbine site, which sits downstream 20 feet or so. Going farther downstream and running another 30 to 40 feet of pipe would probably gain a few more feet of head. The flow varies, and I will start measuring this via a container method next time I’m at the site.

The dam is a rock-and-earth-silt structure in which a large welded metal box sits. The box has a divider down the middle into which aluminum plates are inserted. Water levels behind the dam are controlled by adding or removing plates.

The discharge side of the plates (or back half of this box) then drops about 8 feet into a culvert attached on the back of the box. This site is in northern Michigan, so cold weather is an issue, but the water runs all the time except during severe drought.

What type of microhydro design should I pursue? Should I capture the overflow in a penstock to a turbine past the discharge culvert? Or can potential energy be captured from the dam itself and the turbine located inside the base of the box on the discharge side? Or should I run an AC system right off the discharge culvert itself?

The dam is certified/permitted by the state of Michigan, so I don’t think installing a microhydro system would be forbidden. My intent is to have an off-grid battery-based system to supply a small camp. Someday, I would like to live there, and microhydro would be a key ingredient to reaching that goal.

Craig Schultz • via e-mail

Your question about using an existing dam for microhydro is a good one. Before discussing your intake question, I’d like to emphasize the importance of accurately determining head and flow before you proceed too far. In the head range of 10 to 15 feet you describe, it takes a lot of water to develop useful power—and every foot of head counts. The simple formula for calculating power at this scale is:

Net head (ft.) x flow (gpm) ÷ 12 = watts x 24 hrs. = Wh/day

If you plan to power a camp with battery storage and an inverter, you can probably get away with as little as 3,600 Wh each day. Thus, you will need a summer flow of at least 180 gpm at 10 feet of head. If you can fill a 5-gallon pail in 1.6 seconds, you could consider this option—180 gpm is just about the maximum flow you can measure with a 5-gallon container; much more than that and the container fills too quickly to time the flow accurately. Perhaps you could use a barrel or tank, or even something larger, to get a more accurate flow measurement. In your situation, much greater flow can be measured using the aluminum divider plate you describe in your overflow as a knife-edged weir. You can find the method and formulae for this technique with a little research on the Web, or in Dan New’s excellent articles in the back issues of Home Power.

The existing overflow was designed to handle expected flows and protect your dam. The earthen-and-stone dam you describe is subject to failure if high flows overtop and erode the dam. For that reason, I never recommend using existing overflow pipes to develop microhydro. The danger of disturbing the balance of flow and discharge area is too great.

You are better off putting in another intake for the turbine by penetrating the dam at one end. The penstock should be located about 2 to 3 feet below the water surface. Create the intake with a concrete box facing into the pond, with a screen covering of at least 4 square feet. Place your turbine and generator off to the side of the stream, and as low as possible. Protect it from high flows with a concrete or block enclosure. Wood will work if you can drive wooden pilings to anchor things. Get some good advice on penetrating the dam and be sure to backfill your excavation with rammed clay. If this isn’t done correctly, you could jeopardize the dam. Always consult an engineer and contact the state before starting work on a dam.

As an example, let’s size your penstock for 200 gpm and 40 feet in length. Using loss tables, a 6-inch PVC pipe will only lose about 2.5 inches of head over 50 feet if it is a straight run. That calculation shows the head loss to be insignificant and that the 6-inch penstock is of sufficient size.

Besides sizing your system to work correctly, you’ll need to have a way to shut it down in case of flooding or system maintenance. One way to stop the system is to drop a piece of steel over the pipe inlet at the intake. Be sure to put in a riser pipe from the penstock to above lake level to let air in when you shut down or you may crush the penstock with atmospheric pressure. If you choose to use a valve, use a gate valve so you have to close it slowly, and keep it at least 5 feet upstream (10 pipe-diameters) from the turbine. This lets the water “straighten out” before it gets to the turbine, since turbulence affects power. Good luck, and most of all, have a good time.

Comments (2)

Need help: been searching for 2 weeks online for a micro hydro system that I can use on my site which has the following:
(1) flow rate=5liters/second
(2) head=1000m
(3) voltage used@ house=240V.
(4) Transmission distance=3km

The head can be trimmed to 500m but I want to use the maximum head
that the site is capable of providing thus increasing the power output from such a small flow rate.

Any advice specifically on what and where I can buy a system that will fit the above parameters for my site will really help me stop searching...(sigh!)...